The present invention relates to ionic separation processes and, more particularly, to a method of selectively separating ions using a reverse osmosis (RO) membrane operating at a sub-osmotic pressure.
In a conventional RO separation process, a RO membrane is used to separate a feed solution into a permeate, which carries significantly reduced dissolved material (solute), and a reject salt solution which carries away the bulk of the solute. This is achieved by applying a pressure to the feed solution, in excess of the feed solution's osmotic pressure. Such a process yields a reject solution with essentially the same solute constituents as the feed solution. By contrast, in theory, applying less pressure than the feed solution's osmotic pressure would impede permeate generation.
The present inventor has recognized a need for improved methods for separating multivalent positive ions (cations) from solutions where the solute is principally composed of alkali cations and halide anions.